Oxidizing method and oxidation system

ABSTRACT

An oxidation method of oxidizing surfaces of workpieces heated at a predetermined temperature in a vacuum atmosphere in a processing vessel produces active hydroxyl and active oxygen species. The active hydroxyl and active oxygen species oxidize the surfaces of the workpieces in a processing vessel. Both the intrafilm thickness uniformity and the characteristics of the oxide film can be improved, maintaining oxidation rate on a relatively high level.

TECHNICAL FIELD

[0001] The present invention relates to an oxidation method of oxidizingsurfaces of workpieces, such as semiconductor wafers, and an oxidationsystem.

BACKGROUND ART

[0002] Generally, a semiconductor wafer, such as a silicon substrate, issubjected to various processes including a film forming process, anetching process, an oxidation process, a diffusion process and amodification process when fabricating a semiconductor integratedcircuit. For example, the oxidation process among those processes isused for oxidizing a surface of a single-crystal silicon film or apolysilicon film and for oxidizing a metal film. The oxidation processis used mainly for forming gate oxide films and insulating films forcapacitors.

[0003] Oxidation methods are classified by pressure into atmosphericpressure oxidation methods that are carried out in an atmosphericatmosphere and vacuum oxidation methods that are carried out in a vacuumatmosphere. Oxidation methods are classified by oxidizing gas into wetoxidation methods including a wet oxidation method disclosed in, forexample, JP-A No. Hei 3-140453, that use steam generated by burninghydrogen in an oxygen atmosphere in an external combustor, and dryoxidation methods including a dry oxidation method disclosed in, forexample, JP-A No. Sho 57-1232 that supply only ozone or oxygen into aprocessing vessel.

[0004] In view of quality and characteristics including dielectricstrength, corrosion resistance and reliability, an insulating filmformed by a dry oxidation process is superior to that formed by a wetoxidation process. In view of deposition rate and uniformity, generally,an oxide film (insulating film) formed by an atmospheric oxidationprocess is satisfactory in oxidation rate but the same is notsatisfactory in the intrafilm thickness uniformity of an oxide layerformed on the surface of the wafer. On the other hand, an oxide filmformed by a vacuum oxidation process is satisfactory in the intrafilmthickness uniformity of the oxide layer but the same is not satisfactoryin oxidation rate.

[0005] Design rules that have been hitherto applied to designingsemiconductor integrated circuits have not been very severe theaforesaid various oxidation methods have been selectively used takinginto consideration purposes of oxide films, process conditions andequipment costs. However, line width and film thickness have beenprogressively decreased and severer design rules have been applied todesigning semiconductor integrated circuits in recent years, and designrules requires higher film characteristics and higher intrafilmthickness uniformity of films. The conventional oxidation methods areunable to meet such requirements satisfactorily.

[0006] A wet oxidation system disclosed in, for example, JP-A No. Hei4-18727 supplies H₂ gas and O₂ gas individually into a lower region in avertical quartz processing vessel, burns the H₂ gas in a combustionspace defined in a quartz cap to produce steam, makes the steam flowupward along a row of wafers to accomplish an oxidation process.

[0007] Since this prior art oxidation system burns H₂ gas in thecombustion space, a lower end region in the processing vessel has a highsteam concentration, the steam is consumed as the same flows upward andan upper end region in the processing vessel has an excessively lowsteam concentration. Accordingly, the thickness of an oxide film formedon the surface of the wafer is greatly dependent on the position wherethe wafer is held for the oxidation process and, in some cases, theintrafilm thickness uniformity of the oxide film is deteriorated.

[0008] Another oxidation system disclosed in, for example, JP-A No. Sho57-1232 arranges a plurality of wafers in a horizontal batch-processingreaction tube, supplies O₂ gas or supplies O₂ gas and H₂ gassimultaneously through one of the opposite ends of the reaction tubeinto the reaction tube., and forms an oxide film in a vacuum atmosphere.

[0009] However, since this prior art oxidation system forms a film in anatmosphere of a relatively high pressure by a hydrogen burning oxidationmethod, steam is a principal element of reaction, an upper region withrespect to the flowing direction of gases in the processing vessel and alower region in the processing vessel differ excessively from each otherin steam concentration and hence it is possible that the intrafilmthickness uniformity of the oxide film is deteriorated.

[0010] A third oxidation system disclosed in, for example, U.S. Pat. No.6,037,273 supplies O₂ gas and H₂ gas into the processing chamber of awafer-fed processing vessel provided with a lamp heating device, makesthe O₂ gas and the H₂ gas interact in the vicinity of the surface of asemiconductor wafer placed in the processing chamber to produce steam,and forms an oxide film by oxidizing the surface of the wafer with thesteam.

[0011] However, this oxidation system supplies O₂ gas and H₂ gas throughgas inlets spaced a short distance in the range of 20 to 30 mm from thewafer into the processing chamber, makes the O₂ gas and the H₂ gasinteract in the vicinity of the surface of the semiconductor wafer toproduce steam, and forms the oxide film in an atmosphere of a relativelyhigh process pressure. Thus, it is possible that the intrafilm thicknessuniformity of the film is deteriorated.

DISCLOSURE OF THE INVENTION

[0012] The present invention has been made to solve the aforesaidproblems effectively. Accordingly, it is an object of the presentinvention to provide an oxidation method and an oxidation system capableof improving the intrafilm thickness uniformity of the oxide film andthe interfilm thickness uniformity of oxide films and thecharacteristics of oxide films, maintaining oxidation rate on arelatively high level.

[0013] According to the present invention, an oxidation method ofoxidizing surfaces of workpieces heated at a predetermined temperaturein a vacuum atmosphere created within a processing vessel comprises thesteps of: producing active hydroxyl species and active oxygen species;and oxidizing the surfaces of the workpieces by the active hydroxyl andthe active oxygen species.

[0014] In the oxidation method according to the present invention, anoxidative gas and a reductive gas are supplied into the processingvessel by separate gas supply systems, respectively, in the step ofproducing active hydroxyl and active oxygen species.

[0015] In the oxidation method according to the present invention, theprocessing vessel has a predetermined length, the workpieces are arrangeat predetermined pitches in a processing region in the processingvessel, an oxidative gas and a reductive gas are supplied into theprocessing vessel so as to flow from one end of opposite ends of theprocessing vessel toward the other end of the processing vessel in thestep of producing active hydroxyl and active oxygen species.

[0016] In the oxidation method according to the present invention, partsof the processing vessel through which the oxidative gas and thereductive gas are supplied into the processing vessel are positioned apredetermined distance apart from the processing region of theworkpieces in the processing vessel.

[0017] In the oxidation method according to the present invention, thepredetermined distance is determined such that the oxidative gas and thereductive gas do not affect adversely temperature distribution in theprocessing region of the workpieces and the oxidative gas and thereductive gas supplied into the processing vessel can be satisfactorilymixed.

[0018] The separation of the parts of the processing vessel throughwhich the oxidative gas and the reductive gas are supplied into theprocessing vessel from the processing region by the predetermineddistance prevents the oxidative gas and the reductive gas from adverselyaffecting temperature distribution in the processing region in which theworkpieces are processed and enables the satisfactory mixing of theoxidative gas and the reductive gas.

[0019] In the oxidation method according to the present invention, thepredetermined distance is about 100 mm or above.

[0020] In the oxidation method according to the present invention, theoxidative gas contains one or some of O₂, N₂O, NO and NO₂, and thereductive gas contains one or some of H₂, NH₃, CH₄ and HCl.

[0021] Both the intrafilm thickness uniformity and the characteristicsof the oxide film can be improved, maintaining oxidation rate on arelatively high level.

[0022] In the oxidation method according to the present invention, thepressure in the vacuum atmosphere is below 133 Pa (1 Torr).

[0023] In the oxidation method according to the present invention, thepressure in the vacuum atmosphere is in the range of 6.7 to 67 Pa (0.05to 0.5 Torr).

[0024] In the oxidation method according to the present invention, thepredetermined temperature is in the range of 400 to 1100° C.

[0025] In the oxidation method according to the present invention, anadditional oxidative gas and an additional reductive gas are suppliedadditionally into the processing vessel so as to flow in an oppositedirection of the main oxidation gas and the main reductive gas in thestep of producing active hydroxyl and active oxygen species.

[0026] An oxidation system according to the present invention comprises:a processing vessel for containing workpieces; a support means forsupporting workpieces in a processing region in the processing vessel; aheating means for heating workpieces; an evacuation system forevacuating the processing vessel; an oxidative gas supply system forsupplying an oxidative gas into the processing vessel; and a reductivegas supply system separate from the oxidative gas supply system, forsupplying a reductive gas into the processing vessel to produce activehydroxyl and active oxygen species by the interaction of the oxidativegas and the reductive gas; wherein surfaces of workpieces placed in theprocessing region are oxidized by the active hydroxyl and the activeoxygen species.

[0027] In the oxidation system according to the present invention, theoxidative gas supply system and the reductive gas supply system areconnected to one end of the processing vessel to make the oxidative gasand the reductive gas flow toward the other end of the processingvessel.

[0028] In the oxidation system according to the present invention, theheating means heats both the oxidative gas and the reductive gas.

[0029] In the oxidation system according to the present invention, theoxidative gas supply system has an oxidative gas supply nozzle, thereductive gas supply system has a reductive gas supply nozzle, theoxidative gas supply nozzle and the reductive gas supply nozzle haveoutlets positioned a predetermined distance apart from the processingregion of the workpieces.

[0030] In the oxidation system according to the present invention, thepredetermined distance is determined such that the oxidative gas and thereductive gas do not affect adversely temperature distribution in theprocessing region of the workpieces and the oxidative gas and thereductive gas can be satisfactorily mixed.

[0031] The separation of the outlets of the oxidative gas supply nozzleand the reductive gas supply nozzle from the processing region by thepredetermined distance prevents the oxidative gas and the reductive gasfrom adversely affecting temperature distribution in the processingregion in which the workpieces are processed and enables thesatisfactory mixing of the oxidative gas and the reductive gas.

[0032] In the oxidation system according to the present invention, thepredetermined distance is about 100 mm or above.

[0033] In the oxidation system according to the present invention, theoxidative gas supply system has an oxidative gas supply nozzle, thereductive gas supply system has a reductive gas supply nozzle, and boththe nozzles extend from one of the opposite ends of the processingvessel toward the other end of the processing vessel and have gasoutlets positioned at the other end of the processing vessel

[0034] In the oxidation system according to the present invention, theoxidative gas contains one or some of O₂, N₂O, NO and NO₂, and thereductive gas contains one or some of H₂, NH₃, CH₄ and HCl.

[0035] In the oxidation system according to the present invention, theoxidative gas supply system has a supplementary oxidative gas supplynozzle, the reductive gas supply system has a supplementary reductivegas supply nozzle, and the supplementary oxidative gas supply nozzle andthe supplementary reductive gas supply nozzle have gas outlets disposedat one end of the processing vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a schematic view of an example of an oxidation systemthat carries out a film forming method in a preferred embodimentaccording to the present invention;

[0037]FIG. 2 is a graph showing the failure ratio of SiO₂ films formedby an oxidation method according to the present invention and aconventional oxidation method (dry oxidation method);

[0038]FIG. 3 is a graph showing the distribution of film thicknessdifferences each between a maximum thickness and a minimum thicknessesof each of SiO₂ films formed by an oxidation method according to thepresent invention and those each between a maximum thickness and aminimum thickness of each of SiO₂ films formed by a conventionaloxidation method (external combustion type atmospheric wet oxidationmethod);

[0039]FIG. 4 is a graph showing the variation of the thickness of oxidefilms with oxidation time;

[0040]FIG. 5 is a graph showing the dependence of the thickness andintrafilm thickness uniformity of oxide films on process pressure;

[0041]FIG. 6 is a graph showing thicknesses of oxide films in a pressurerange including pressures below the pressures shown in FIG. 5;

[0042]FIG. 7 is a schematic view of a oxidation system of single-tubestructure;

[0043]FIG. 8 is a schematic view of another oxidation system ofsingle-tube structure; and

[0044]FIG. 9 is a schematic view of a third oxidation system ofsingle-tube structure.

BEST MODE FOR CARRYING OUT THE INVENTION

[0045] An oxidation method and an oxidation system in preferredembodiments according to the present invention will be describedhereinafter with reference to the accompanying drawings.

[0046]FIG. 1 shows an example of an oxidation system 2 that carries outa film forming method in a preferred embodiment according to the presentinvention. In the following description, it is assumed that an oxidativegas is oxygen gas (O²) and a reductive gas is hydrogen gas (H₂).

[0047] The oxidation system 2 has a double-wall vertical processingvessel 8 of a predetermined length including an inner tube 4 and anouter tube 6. Both the inner tube 4 and the outer tube 6 are made ofquartz. A quartz wafer boat 10, i.e., a support means for holdingworkpieces, is placed in a processing space in the inner tube 4. Aplurality of semiconductor wafers W are held in layers at predeterminedpitches on the wafer boat 10. The pitches are equal for some cases andare different for other cases.

[0048] A cap 12 covers the open lower end of the processing vessel 8,and a shaft 16 is extended through the cap 12. The gap between the cap12 and the shaft 16 is sealed by a magnetic fluid seal 14, and a rotarytable 18 is attached to the upper end of the shaft 16. A heat insulatingtube 20 is placed on the rotary table 18 and the wafer boat 10 ismounted on the heat insulating tube 20. The shaft 16 is supported on anarm 24 included in a vertically movable boat elevator 22. The shaft 16can be moved vertically together with the cap 12 and the wafer boat 10.The wafer boat 10 can be inserted in and taken out of the processingvessel 8 through the lower end of the processing vessel 8. The waferboat 10 does not need necessarily to be rotated and may be heldstationary.

[0049] A manifold 26 made of, for example, a stainless steel is joinedto the open lower end of the processing vessel 8. An oxidative gassupply system 28 for supplying an oxidative gas at a controlled flowrate and a reductive gas supply system 30 for supplying a reductive gasat a controlled flow rate are connected individually to the manifold 26.

[0050] The oxidative gas supply system 28 includes an oxidative gassupply nozzle 32 penetrating the manifold, and a gas supply line 36connected to the oxidative gas supply nozzle 32 and provided with a flowcontroller 34, such as a mass flow controller. An oxidative gas source30 storing an oxidative gas, such as oxygen gas in this embodiment, isconnected to the gas supply line 36.

[0051] The reductive gas supply system 30 includes a reductive gassupply nozzle 40 penetrating the manifold 26, and a gas supply line 44connected to the reductive gas supply nozzle 40 and provided with a flowcontroller 42, such as a mass flow controller. A reductive gas source 46storing a reductive gas, such as hydrogen gas in this embodiment, isconnected to the gas supply line 44.

[0052] The gas supply nozzles 32 and 40 have gas outlets 32 a and 40 a,respectively. The gas outlets 32 a and 40 a are formed in the manifold26 disposed at one end of the processing vessel 8.

[0053] The gases supplied through the gas outlets 32 a and 40 a of thegas supply nozzles 32 and 40 flow from one end to the other end of theprocessing vessel 8. That is, the gases flow upward in the processingspace S, i.e., a wafer processing region, in the inner tube 4, startflowing down from a top of the processing vessel 8, flow down through anspace between the inner tube 4 and the outer tube 6 and flow out of theprocessing vessel 8. An exhaust port 50 is formed in a lower part of theside wall of the outer tube 6. An evacuating system 56 including anexhaust line 52 and a vacuum pump 54 connected to the exhaust line 52 isconnected to the exhaust port 50 to evacuate the processing vessel 8.The processing space S, i.e., a wafer processing region, is positioned apredetermined distance H1 apart from the position of a gas supplyposition. More concretely, the lower end of the processing space Scorresponding to the lower end of the wafer boat 10 is positioned apredetermined distance H1 apart from the positions of the respective gasoutlets 32 a and 40 a of the nozzles 32 and 40. A first purpose of thedistance H1 is to preheat the gases by heat radiated by the hot wall ofthe processing vessel 8 heated by a heater 62 while the gases flowupward through the distance H1. Generally, the processing space Sextending along the wafer boat 10 is maintained accurately at a fixedtemperature. If gases of, for example, a room temperature is supplied toa region in the vicinity of a lower end part of the wafer boat 10, thetemperature of the same region will drop, adversely affectingtemperature distribution in the processing space. A second purpose ofthe distance H1 is to enable the gases to mix uniformly while the gasesflow upward through the distance H1.

[0054] The distance H1 is determined such that the oxidative gas and thereductive gas supplied into the processing space S do not affectadversely to temperature distribution in the processing space S, and mixuniformly. The distance H1 is, for example 100 mm or above, preferably,300 mm or above. In this embodiment, the distance H1 is about 350 mm. Aheat insulating member 60 is disposed so as to surround the processingvessel 8, and the heater 62, i.e., a heating means, is attached to theinner circumference of the heat insulating member 60 to heat wafers Wplaced in the processing space S at a predetermined temperature.

[0055] When, for example, about 150 wafers W of 8 in. diameter (about130 wafers and about 20 dummy wafers) supported on the wafer boat 10 areprocessed in a batch, the inner tube 4 of the processing vessel 8 isabout 260 to about 270 mm in diameter, the outer tube 6 is about 275 toabout 285 mm in diameter and the processing vessel 8 is about 1280 mm inheight.

[0056] When about 25 to 50 wafers w of 12 in. diameter supported on thewafer boat 10 are processed in a batch the inner tube 4 is about 380 toabout 420 mm, the outer tube 6 is about 440 to about 500 mm in diameterand the processing vessel is about 800 mm in height. The height of thewafer boat 10 is equal to the height H2 of the processing space S2 inwhich the wafers are placed for processing. The height H2 is dependenton the number of wafers to be processed in a batch and is, for example,in the range of about 200 to about 1000 mm. Those numerical values aregiven only by way of example.

[0057] Shown in FIG. 1 are a sealing member 64, such as an O ring, forsealing the gap between the cap 12 and the manifold 26, a sealing member66, such as an O ring, for sealing the gap between the manifold 26 andthe lower end of the outer tube 6.

[0058] An oxidizing method according to the present invention to becarried out by the aforesaid oxidation system will be described.

[0059] A plurality of semiconductor wafers W are supported in layers atpredetermined pitches on the wafer boat 10. The boat elevator 22 ismoved upward to insert the wafer boat 10 through the lower end of theprocessing vessel 8 in the processing vessel 8 and the processing vessel8 is sealed hermetically. The processing vessel 8 is heated beforehand.Films to be oxidized, such as single crystal films, polysilicon films ormetal oxide films, have already formed on surfaces of the semiconductorwafers W, respectively, by the preceding process. In some cases,surfaces of single-crystal wafers are oxidized.

[0060] After the wafers W have been loaded into the processing vessel 8,power supplied to the heater 62 is increased to heat the wafers W at apredetermined processing temperature and the processing vessel 8 isevacuated by the evacuating system 56.

[0061] At the same time, the oxidative gas supply system 28 suppliesoxygen gas at a controlled flow rate through the gas outlet 32 a of theoxidative gas supply nozzle 32 and the reductive gas supply system 30supplies hydrogen gas at a controlled flow rate through the gas outlet40 a of the reductive gas supply nozzle 40 into the processing vessel 8.

[0062] The oxygen gas and the hydrogen gas separately supplied into theprocessing vessel 8 flow upward in the processing vessel 8, whileproducing active hydroxyl species and active oxygen species to oxidizesurfaces of the wafers W. When oxidizing single-crystal silicon films orpolysilicon films, the temperature of the wafers W is in the range of400 to 1100° C., preferably, in the range of 400 to 900° C. taking intoconsideration the heat resistance of the wafers is taken intoconsideration, the pressure in the processing vessel 8 is 133 Pa (1Torr) or below, preferably, in the range of 6.7 Pa (0.05 Torr) to 67 Pa(0.5 Torr) taking into consideration concentration distribution. Oxygengas flow rate is in the range of 1 sccm to 10 slm and oxygen gas flowrate is in the range of 1 sccm to 5 slm.

[0063] Thus, oxidation rate is maintained on a relatively high level andthe intrafilm thickness uniformity and the quality of an oxide film oneach wafer W can be improved greatly. Uniformity of plurality of wafersW in film quality can be also improved.

[0064] When oxygen gas and hydrogen gas are supplied separately into avacuum atmosphere of the processing vessel 8, it is considered that thehydrogen gas undergoes the following combustion reactions, in whichchemical symbols with asterisk (*) indicates an active specie of asubstance indicated by the chemical symbol.

H₂+O₂→H*+HO₂

O₂+H*→OH*+O*

H₂+O*→H*+OH*

H₂+OH*→H*+H₂O

[0065] When hydrogen gas (H₂) and oxygen gas (O₂) are suppliedseparately into the processing vessel 8, active oxygen species (O*),active hydroxyl species (OH*) and steam (H₂O) are produced by theburning of hydrogen gas. It is inferred that O* and OH* contributesgreatly to the improvement of he film quality and the intrafilmthickness uniformity of the oxide film. That is, since the oxidationprocess is carried out in a vacuum atmosphere of a pressure which is farbelow a process pressure of the conventional oxidation method, theaforesaid reactions represented by the chemical formulas proceedsgradually while oxygen gas and hydrogen gas flows upward in theprocessing vessel 8, proper amounts of H₂O, O* (active oxygen species)and OH* (active hydroxyl species) are supplied to all the wafers Wregardless of the height of the wafers W. Consequently, all the wafers Ware subjected uniformly to the oxidation process regardless of theheight thereof and the film thickness uniformity of the oxide filmsformed on the wafers W can be improved. It is considered that activeoxygen species and active hydroxyl species contribute greatly to theoxidation of the wafers W and the life of the active oxygen species andactive hydroxyl species is extended by carrying out the oxidation methodin the vacuum atmosphere of a process pressure considerably lower thanthat at which the conventional oxidation method is carried out.Therefore, the active species do not become very extinct and activespecies concentration in the processing space S is maintained in asubstantially uniform manner while the active species flow upwardthrough the processing space S of the height H2 contributing to theoxidation reaction. Consequently, the thickness uniformity of filmsrespectively formed on the wafers W placed at different heights can beimproved.

[0066] Moreover, since the life of the oxygen and hydrogen species isextended, active species concentration distribution over the surface ofeach wafer W, from the periphery edge to the center thereof, is uniformand hence the intrafilm thickness uniformity and the characteristics ofthe oxide film can be improved greatly.

[0067] Since oxygen gas and hydrogen gas are supplied into the positionwhich is located distance H1 apart from the lower end of the processingspace S instead of supplying the same directly into the processing spaceS, the gases mix satisfactorily and are preheated by the heat radiatedby the hot wall of the processing vessel 8 heated by the heater 62 whilethe same flow through the distance H1, which promotes the activation ofthe gases.

[0068] Silicon films were oxidized by the oxidation method of thepresent invention and a conventional dry oxidation method to formsilicon dioxide films (SiO₂ films) and the characteristics of thesilicon dioxide films were examined. The results of the examination willbe explained.

[0069]FIG. 2 shows the failure ratio of SiO₂ films formed by theoxidation method of the present invention and the conventional dryoxidation method. Time that was necessary to cause 90% of the SiO₂ filmsto break down by forcibly passing a current of 0.05 A/cm² through eachSiO₂ films was measured.

[0070] As is obvious from FIG. 2, whereas 90% of the SiO₂ films formedby the conventional wet oxidation method broke down in about 6 s, timenecessary to cause 90% of the SiO₂ films formed by the oxidation methodof the present invention broke down was as long as about 50 s, whichproved the excellent dielectric strength and reliability andsatisfactory quality of the SiO₂ films formed by the oxidation method ofthe present invention. A total amounts of charge that was necessary tocause 90% of the SiO₂ films formed by the conventional wet oxidationmethod and the SiO₂ films formed by the oxidation method of the presentinvention broke down were 0.25 C/cm² and 2.35 C/cm² respectively.

[0071] Silicon films were oxidized by the oxidation method of thepresent invention and a conventional external combustion typeatmospheric wet oxidation method to form silicon dioxide films (SiO₂films) and the intrafilm thickness uniformity of the SiO₂ films wereexamined. The results of the examination will be explained.

[0072]FIG. 3 is a graph showing the distribution of film thicknessdifferences, each between a maximum thickness and a minimum thickness ofeach of SiO₂ films formed by the oxidation method of the presentinvention, and those, each between a maximum thickness and a minimumthickness of each of SiO₂ films formed by the conventional externalcombustion type atmospheric wet oxidation method. Process conditions forthe oxidation method of the present invention were 850° C. in processtemperature, 26.6 Pa (0.2 Torr) in process pressure, 0.1 slm in O₂ gasflow rate and 0.2 μm in H₂ gas flow rate. Process conditions for theconventional oxidation method were 850° C. in process temperature, 95760Pa (720 Torr) in process pressure, 0.6 slm in O₂ gas flow rate, 0.6 slmin H₂ gas flow rate and 20 slm in N₂ gas flow rate. The surfaces ofwafers were oxidized to form oxide films respectively having thicknessesin the range of 1 to 4 nm.

[0073] As is obvious from the graph shown in FIG. 3, whereas thethickness difference of the oxide films on each wafer formed by theconventional oxidation method is distributed in a wide range regardlessof the thickness of the oxide film, the thickness difference of theoxide film on each wafer formed by the oxidation method of the presentinvention is distributed in a narrow range. Thus the intrafilm thicknessdifferences of the oxidation films formed by the oxidation method of thepresent invention are considerably small. The mean thickness differenceof the oxide films formed by the conventional oxidation method was 0.066nm and that of the oxide films formed by the oxidation method of thepresent invention was 0.047 nm, which proved that the oxidation methodof the present invention improves the intrafilm thickness uniformitygreatly.

[0074] Oxidation rate of the oxidation method of the present inventionand that of a conventional oxidation method that supplies steam directlyinto the processing vessel were examined.

[0075]FIG. 4 is a graph showing the relation between oxidation time andthe thickness of oxide films. Process conditions were 850° C. in processtemperature, 93 Pa (0.7 Torr) is process pressure, 100 sccm in H₂ gasflow rate and 600 sccm in O₂ gas flow rate.

[0076] As is obvious from The graph shown in FIG. 4, the thickness of anoxide film formed by the oxidation method of the present invention isten times or above that of an oxide film formed by the conventionaloxidation method in the same time; that is, the oxidation rate of theoxidation method of the present invention is ten times or above that ofthe conventional oxidation method, and hence the oxidation method of thepresent invention increases the throughput of the oxidation processaccordingly.

[0077] The dependence of the thickness of oxide films on processpressure was examined. The result of the examination will be explained.

[0078]FIG. 5 is a graph showing the dependence of the thickness andintrafilm thickness uniformity of oxide films formed on the wafer whichis located at the same position in the same batch, on process pressure.FIG. 6 is a graph showing the dependence of the thickness of oxidefilms, on process pressure in a pressure range including pressures belowthe pressures shown in FIG. 5. In FIG. 5, values indicating interfilmthickness uniformity of different wafers W are written along with curvesindicating the thicknesses of oxide films. In the graphs shown in FIGS.5 and 6, TOP, CTR and BTM indicate wafers placed at upper, middle andlower positions, respectively, on the wafer boat 10. Process conditionsfor forming the oxide films shown in FIGS. 5 and 6(A) are 900° C. inprocess temperature, 600 sccm in H₂ gas flow rate, 1200 sccm in O₂ gasflow rate and 60 min in processing time. Process conditions for formingthe oxide films shown in FIG. 6(B) are 850° C. in process temperature,50 sccm in H₂ gas flow rate, 100 sccm in O₂ gas flow rate and 3 min inprocessing time.

[0079] As is obvious from FIG. 5, the lower the process pressure is, thebetter both the intrafilm thickness uniformity and the interfilmthickness uniformity. It was found that the process pressure must bebelow 133 Pa (1 Torr) to reduce the intrafilm thickness uniformity to anappropriate value, such as about ±0.8%, that is expected to be achievedby the future oxidation processes.

[0080]FIG. 6(A) is a graph showing, for confirming the reproducibilityof the oxidation method, the dependence of the thickness of oxide filmsformed under the same process conditions as those mentioned in theforegoing description made in connection with FIG. 5, on processpressure, except that process conditions for forming the oxide filmsshown in FIG. 6(A) include a process pressure of 67 Pa (0.5 Torr). It isknown from the comparative examination of the graphs shown in FIGS. 5and 6(A) that modes of variation of the thickness of the oxide film withprocess pressure in FIGS. 5 and 6(A) are substantially the same exceptfor a process condition of a process pressure of 67 Pa, which provesexcellent reproducibility of the oxidation method. FIGS. 5 and 6(A) showthe variation of the thickness on the order of 12 nm of the oxide filmswith process pressure. FIG. 6(B) is a graph showing the variation of thethickness of oxide films with process pressure in a process pressurerange not higher than 67 Pa (0.5 Torr).

[0081] Process conditions shown in FIG. 6(B) are for forming oxide filmsof a desired thickness of 2 nm, which is ⅙ of a desired thickness of 12nm for the oxide films formed under the process conditions shown inFIGS. 5 and 6(A). As is obvious from FIG. 6(B), the thicknesses of theoxide films are on the order of 2 nm for the process pressure range of6.7 Pa (0.05 Torr) to 67 Pa (0.5 Torr) and the thicknesses of the oxidefilms formed on TOP, CTR and BTM are scarcely different from each other.

[0082] It is known from FIG. 6(B) that the oxide films can be formed insatisfactory thickness uniformity even if the oxide films are as thin as2 nm. It is also known from FIG. 6(B) that the thickness uniformity canbe maintained on a high level in the process pressure range of 6.7 Pa(0.05 Torr) to 67 Pa (0.5 Torr).

[0083] It is considered that intrafilm thickness uniformity andinterfilm thickness uniformity can be improved when the process pressureis in the low process pressure range because the life of active oxygenspecies and active hydrogen species produced in the processing vessel 8is extended sufficiently when the process pressure is very low. Thus,when active oxygen species and active hydrogen species flow through theprocessing space S, active species concentration distributions can beuniform throughout the processing space S.

[0084] Oxidation rate can be properly changed and controlled by changingthe flow rate ratio between H₂ gas and O₂ gas or by properly mixing aninert gas, such as nitrogen gas, argon gas or helium gas, into H₂ gasand O₂ gas.

[0085] Although the present invention has been described as applied tothe oxidation system provided with the processing vessel 8 ofdouble-wall construction, the present invention is applicable also to aprocessing system provided with a processing vessel of single-wallconstruction. In the processing system provided with the processingvessel of single-wall construction, gases may be supplied into theprocessing vessel so as to flow from the upper end of the processingvessel toward the lower end of the processing vessel as shown in FIG. 7.An oxidation system provided with a processing vessel of a single-wallconstruction will be described by way of example with reference to FIG.7, in which parts like or corresponding to those shown in FIG. 1 aredenoted by the same reference characters and the description thereofwill be omitted.

[0086] The oxidation system shown in FIG. 7 is provided with aprocessing vessel 8 of single-wall construction provided only with anouter tube 6 and not provided with any member corresponding to the innertube 4 shown in FIG. 1. An exhaust port 50 is formed in a manifold 26.An oxidative gas supply system 28 has an oxidative gas supply nozzle 32attached to a top part of the processing vessel 8 and a reductive gassupply system 30 has a reductive gas supply nozzle 40 attached to thetop part of the processing vessel 8. O₂ gas and H₂ gas are suppliedthrough the respective gas outlets 32 a and 40 a of the nozzles 32 and40, respectively, into the processing vessel 8, and flow down throughthe processing vessel 8. The H₂ gas is burnt to oxidize wafers W and thegases are discharged through the exhaust port 50 by suction.

[0087] An oxidation system as shown in FIG. 8 may be used. The oxidationsystem shown in FIG. 8 is provided with a processing vessel 8 ofsingle-wall construction. An exhaust port 70 is formed in a top part ofthe processing vessel 8. An oxidative gas and a reductive gas may besupplied into the processing vessel 8 through the gas outlets 32 a and40 a of nozzles 32 and 40 attached to a lower part of the processingvessel 8, respectively.

[0088] An oxidation system as shown in FIG. 9 may be used. The oxidationsystem shown in FIG. 9 is provided with a processing vessel 8 of asingle-wall construction. A manifold 26 is joined to the lower end ofthe processing vessel 8. Nozzles 32 and 40 extend through the manifold26 and along the inner surface of the processing vessel 8 to the upperend of the processing vessel 8 so that the gas outlets 32 a and 40 athereof are positioned in a top part of the processing vessel. 8. O₂ gasand H₂ gas are supplied through the gas outlets 32 a and 40 a into theprocessing vessel 8. The gases are activated as the same flow downthrough the processing vessel 8 and are discharged through an exhaustport 50 formed in a lower part of the processing vessel 8.

[0089] In this embodiment, the gases are preheated sufficiently by heatradiated by a heater 62 and heat radiated by the wall of the processingvessel 8 heated by the heater 62 while the same flow through the nozzles32 and 43 extended along the side wall of the processing vessel 8. Thusactive species of those gases can be more efficiently produced.

[0090] The oxidation system shown in FIG. 9 may be provided with anadditional oxidative gas supply nozzle 32 b and an additional reductivegas supply nozzle 40 b connected to the manifold 26 so that the gasoutlets 32 c and 40 c thereof opens into a lower end region of aprocessing space in the processing vessel 8. An additional oxidative gasand an additional reductive gas can be additionally supplied into theprocessing vessel 8 through the additional oxidative gas supply nozzle32 b and the additional reductive gas supply nozzle 40 b.

[0091] The additional oxidative gas and the additional reductive gasfrom the gas outlet 32 c and 40 c flow in an opposite direction of themain oxidative gas and the main reductive gas from the gas outlets 32 aand 40 a.

[0092] Although the aforesaid embodiment uses O₂ gas as an oxidative gasand uses H₂ gas as a reductive gas, the oxidative gas may be one or someof O₂, N₂O, NO and NO₂, and the reductive gas may be one or some of H₂,NH₃, CH₄ and HCl.

[0093] Principally, active oxygen species and active hydroxyl speciesproduced by the burning of the reductive gas contribute to an oxidationreaction that occurs on the surfaces of the wafers. When gases otherthan O₂ gas and H₂ gas are used, process conditions including thetemperature of the wafers and process pressure may be similar to thosewhen oxygen gas and hydrogen gas are used.

[0094] The oxidation method according to the present invention isapplicable not only to the foregoing batch-processing oxidation systemcapable of oxidizing a plurality of wafers in a batch, but also to asingle-wafer processing oxidation system that supports a semiconductorwafer on a stage (support means) placed in a processing vessel andprocesses the single wafer for oxidation by heating the semiconductorwafer by a heating means, such as lamps of a heater.

[0095] The workpieces are not limited to semiconductor wafers and may besubstrates for LCDs and glass substrates.

[0096] As apparent from the foregoing description, the oxidation methodand the oxidation system according to the present invention exercise thefollowing excellent effects.

[0097] Intrafilm uniformity, interfilm uniformity and film quality ofoxide films can be improved maintaining oxidation rate on a relativelyhigh level.

What is claimed is:
 1. An oxidation method of oxidizing surfaces ofworkpieces heated at a predetermined temperature in a vacuum atmospherewithin a processing vessel, said oxidation method comprising the stepsof: producing active hydroxyl and active oxygen species; and oxidizingthe surfaces of the workpieces by the active hydroxyl and the activeoxygen species.
 2. The oxidation method according to claim 1, wherein anoxidative gas and a reductive gas are supplied into the processingvessel by separate gas supply systems, respectively, in the step ofproducing active hydroxyl and active oxygen species.
 3. The oxidationmethod according to claim 1 or 2, wherein the processing vessel has apredetermined length, the workpieces are arrange at predeterminedpitches in a processing region in the processing vessel, an oxidativegas and a reductive gas are supplied into the processing vessel so as toflow from one of opposite ends of the processing vessel toward the otherof opposite ends in the step of producing active hydroxyl and activeoxygen species.
 4. The oxidation method according to claim 3, whereinparts of the processing vessel through which the oxidative gas and thereductive gas are supplied into the processing vessel are positioned apredetermined distance apart from the processing region of theworkpieces in the processing vessel.
 5. The oxidation method accordingto claim 4, wherein the predetermined distance is determined such thatthe oxidative gas and the reductive gas do not affect adverselytemperature distribution in the processing region of the workpieces andthe oxidative gas and the reductive gas supplied into the processingvessel can be satisfactorily mixed.
 6. The oxidation method according toclaim 5, wherein the predetermined distance is about 100 mm or above. 7.The oxidation method according to claim 1, wherein the oxidative gascontains one or some of O₂, N₂O, NO and NO₂, and the reductive gascontains one or some of H₂, NH₃, CH₄ and HCl.
 8. The oxidation methodaccording to claim 1, wherein the pressure of the vacuum atmosphere isbelow 133 Pa (1 Torr).
 9. The oxidation method according to claim 8,wherein the pressure of the vacuum atmosphere is in the range of 6.7 to67 Pa (0.05 to 0.5 Torr).
 10. The oxidation method according to claim 1,wherein the predetermined temperature is in the range of 400 to 1100° C.11. The oxidation method according to claim 3, wherein an additionaloxidative gas and an additional reductive gas are supplied additionallyinto the processing vessel so as to flow from in an opposite directionof the oxidative gas and the reductive gas flowing from one end towardthe other end in the step of producing active hydroxyl and active oxygenspecies.
 12. An oxidation system comprising: a processing vessel forcontaining workpieces; a support means for supporting workpieces in aprocessing region in the processing vessel; a heating means for heatingworkpieces; an evacuation system for evacuating the processing vessel;an oxidative gas supply system for supplying an oxidative gas into theprocessing vessel; and a reductive gas supply system separate from theoxidative gas supply system, for supplying a reductive gas into theprocessing vessel to produce active hydroxyl and active oxygen speciesby the interaction of the oxidative gas and the reductive gas; whereinsurfaces of workpieces placed in the processing region are oxidized bythe active hydroxyl and the active oxygen species.
 13. The oxidationsystem according to claim 12, wherein the oxidative gas supply systemand the reductive gas supply system are connected to one end of theprocessing vessel to make the oxidative gas and the reductive gas flowtoward the other end of the processing vessel.
 14. The oxidation systemaccording to claim 13, wherein the heating means heats both theoxidative gas and the reductive gas.
 15. The oxidation system accordingto claim 13 or 14, wherein the oxidative gas supply system has anoxidative gas supply nozzle, the reductive gas supply system has areductive gas supply nozzle, the oxidative gas supply nozzle and thereductive gas supply nozzle have outlets positioned a predetermineddistance apart from the processing region of the workpieces in theprocessing vessel.
 16. The oxidation system according to claim 15,wherein the predetermined distance is determined such that the oxidativegas and the reductive gas do not affect adversely temperaturedistribution in the processing region of the workpieces and theoxidative gas and the reductive gas can be satisfactorily mixed.
 17. Theoxidation system according to claim 16, wherein the predetermineddistance is about 100 mm or above.
 18. The oxidation system according toclaim 12, wherein the oxidative gas supply system has an oxidative gassupply nozzle, the reductive gas supply system has a reductive gassupply nozzle, both the nozzles extend from one end of the processingvessel toward the other end of the processing vessel and have gasoutlets positioned at the other end of the processing vessel.
 19. Theoxidation system according to claim 12, wherein the oxidative gascontains one or some of O₂, N₂O, NO and NO₂, and the reductive gascontains one or some of H₂, NH₃, CH₄ and HCl.
 20. The oxidation systemaccording to claim 18, wherein the oxidative gas supply system has asupplementary oxidative gas supply nozzle, the reductive gas supplysystem has a supplementary reductive gas supply nozzle, and thesupplementary oxidative gas supply nozzle and the supplementaryreductive gas supply nozzle have gas outlets disposed at one end of theprocessing vessel.